This invention relates to a lead frame, a circuit board for mounting a semiconductor integrated circuit and a semiconductor device as well as their manufacturing methods and, more particularly, to a lead frame, a circuit board for mounting a semiconductor integrated circuit and a semiconductor device as well as their manufacturing methods for use in encapsulating a semiconductor integrated circuit within a mold resin by transfer molding using a mold die which defines a mold cavity when sandwiching the leads therebetween.
A lead frame for use in molding semiconductor elements such as an integrated circuit by a mold resin through the use of a mold die is well known. Such lead frame is formed by cutting an electrically conductive metal sheet to have an electrically conductive frame portion having an opening, an electrically conductive island portion supported by support pins within the frame opening for mounting a semiconductor element thereon and a plurality of lead portions extending from the frame portion toward the island portion. The lead portions are a plurality of electrically conductive lead members spaced apart from each other to define spaces therebetween and having an inner end extending from the frame portion toward the island portion and spaced apart from the island portion, and each lead portion includes an inner lead portion to be encapsulated by the mold resin, an outer lead portion to project outwardly from an outer surface of the mold resin, and a boundary defined between the inner lead portion and the outer lead portion. The lead frame also comprises a tie bar extending between the lead portions for integrally connecting the lead portions together, thereby maintaining a proper position of the lead portions of the frame so that the lead portions are not broken or bent. The tie bar also functions to provide a dam-like barrier for preventing the mold resin from flowing into spaces formed between the lead portions sandwiched between the mold die when the mold die is applied to the lead frame so that the mold resin does not leak from the mold cavity of the mold die upon transfer molding, wherefore the tie bar is also referred to as a dam-bar.
After transfer molding the semiconductor element with the mold resin, the tie-bar must be removed from the lead portions by a suitable punching die so that the respective lead portions are electrically independent from each other. However, since the fineness of the patterns that can be punched out by the punching die is limited, it has not been possible to manufacture a package employing a lead frame having fine pitch leads having a lead pitch of from 0.3 mm to 0.5 mm for example.
Also, since the conductive tie-bar must be completely removed from the package after it is completed, the tie-bar is disposed spaced apart from and outside of the mold resin. Therefore, the mold resin material flows into the spaces between the leads and between the tie-bar and the mold resin side face, so that burrs are formed. While the burrs are formed in the spaces between the number of the leads, they are irregularly and unstably formed such that they appear in some of the spaces while they do not appear in other of the spaces, so that the completed semiconductor device is very much degraded in appearance and its commercial value is reduced. Therefore, the process of manufacturing a semiconductor device includes a step of removing burrs.